BAGHDAD, Jan. 16 -- A U.S. Army helicopter on patrol crashed in a swampy area north of Baghdad on Monday morning, killing both crew members, U.S. authorities said. It was the third such incident this month.

Witnesses said the AH-64 Apache attack helicopter was hit by a missile and came down at a farm near the village of Mishahda, 25 miles north of Baghdad. Two insurgent groups asserted responsibility for attacking the aircraft.

U.S. military authorities in Baghdad said in a statement that it was "premature" to state the cause of the crash, but noted that the helicopter went down in an area "known for terrorist activity." The officials said that the Apache, conducting a combat air patrol, belonged to Task Force Ironhorse and that the incident was under investigation.

The crash area was cordoned off by U.S. troops, and a Washington Post special correspondent was not allowed to approach the scene. A U.S. Army lieutenant, who did not identify himself, said that the helicopter had been hit by a missile and that American troops were searching for the attackers.

Two groups asserted responsibility for the incident. One group, which identified itself as the Mujaheddin Army, published a video on a Web site used by insurgent groups in which there was a whooshing sound followed by a smoke trail as a missile apparently was fired at a speck in the sky. As the stricken helicopter lost altitude, leaving a plume of black smoke, a voice in the background chanted, "Allahu akbar," or "God is great."

The authenticity of the statements and the video could not be verified.

In the two previous incidents, insurgents apparently shot down an OH-58D Kiowa Warrior reconnaissance helicopter on Friday near Mosul in northern Iraq, killing its two pilots. On Jan. 7, a UH-60 Black Hawk helicopter flying in bad weather near the northern city of Tall Afar crashed, killing all four U.S. soldiers and eight American civilians aboard. The military is still investigating both incidents.

The Army's heavily armed and armored Apaches are used to support ground troops with close-in firepower. Although it is unlikely that small-arms fire could bring down an Apache, the helicopter is vulnerable to shoulder-fired, heat-seeking missiles, such as the Russian-made Strela. Thousands of Strelas were sold to the Iraqi government in the years before the U.S. invasion in 2003.

A U.S. military spokesman in Baghdad, Lt. Col. Barry Johnson, said in an interview that it was too early to say what had caused the crash or to find similarities in the three cases.

"I think we take each incident that involves an aircraft going down, or anything else, and make a determination of the circumstances," Johnson said. "Each one is equally tragic, and certainly if there's any pattern or anything related here, that's something we'll consider, and adjust our tactics or techniques."

Authorities reported scattered violence elsewhere across Iraq. A roadside bomb hit a convoy carrying U.S. police liaison officers in Baghdad, killing one American civilian responsible for training Iraqi police, according to a statement issued by the U.S. Embassy.

In Muqdadiyah, about 60 miles north of Baghdad, an explosives-laden car sped into an Iraqi police checkpoint, killing four Iraqi police officers and a child, U.S. military authorities said in a statement. The attack took place near a cluster of primary schools where students were taking their midyear exams, Col. Adnan Lafta of the city police said.